By Robert Hazen, Ph.D., George Mason University
Our world is made up of materials: solids, liquids, and gases. The properties of materials depend on the type of atoms and the way they’re bound together by chemical bonds. Strength is one of the most significant and obvious properties of any material. Material scientists identify three very different kinds of strength.
Materials: Their Properties and Importance
Materials are useful because of their distinctive properties. There are many properties, like color, hardness, flexibility, density, transparency, texture, electrical conductivity, thermal expansion, melting and boiling points, heat capacity, and so on. Indeed, any adjective that you can use to describe a material likely relates in one way or another to its physical properties.
Materials are so important that historic eras of human history have been defined by the major material that was in use. There is the Stone Age, the most ancient age when stone tools and weapons were in prominent use. Then came the Bronze Age. The Bronze Age was at different times in different parts of the world. In the Middle East, where bronze was discovered earlier, the Bronze Age lasted from about 3500 to 1500 B.C., while in Europe, it was more like 2000 to 900 B.C. Next came the Iron Age. The Iron Age began at about 1500 B.C. in the Middle East, and at about 900 B.C. in Europe. These are ages named after materials.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Types of Strength
Material scientists distinguish three very different kinds of strength: there is compressive strength, which is pushing on something; tensile strength, which is pulling it apart; and shear strength, which involves twisting.
Compressive strength refers to a material’s resistance to being crushed. Materials with very high compressive strength have to have a rigid framework of atoms. They have to be strong in three dimensions. For example, a brick, a piece of ceramic, a cinder block, or a rock; they all are extremely strong. They can hold up the weight of thousands of tons if you have a good, strong brick or stone foundation.
Tensile strength is what happens when you pull something apart. Materials with high tensile strength have to have a continuous chain of strongly bonded atoms or molecules; so polymers are particularly effective. For example, a rope, which is extremely strong, can hold hundreds of pounds when pulled because of these chains of carbon atoms that run all the way through.
Shear strength refers to a material’s resistance to being twisted. Materials with high shear strength have to have an incredibly strong cross-bracing in order to prevent twisting, something like a trestle bridge. One of the most obvious examples of material with very high shear strength is diamond.
Learn more about the material world.
When Types of Strength Don’t Go Hand-in-Hand
These three types of material strength, however, don’t always go hand-in-hand. For example, you can have a stack of bricks, which is incredibly strong under compression, but it has virtually no tensile strength unless it’s mortared together. You can push on it, and it’s strong; pull on it, and it’s very weak.
This is a problem with ceramic buildings, buildings made of cinder blocks or bricks, as we’ll see in an earthquake. A nylon rope has exceptional tensile strength—you can apply hundreds of pounds to it—but it has no compressive strength whatsoever. You can’t push on a nylon rope to support anything.
Another example would be in shear strength; a Coke can, for example, is very strong under compression and shear strength when it’s full. If you twist that can, it won’t twist; but as soon as you empty it of liquid, it becomes very, very weak. In many applications, a combination of these three different kinds of strength is required.
Learn more about the organization of atoms and molecules.
Earthquake-proof Buildings and Strength of Material
These considerations come particularly into play in the design of earthquake-proof buildings. An earthquake is a sudden movement of the ground that sends a shockwave through the Earth. The Earth goes through waves: up-and-down, side-to-side. It subjects buildings to terrible stresses. It subjects them to shear stresses, compressive stresses, and tensile stresses, all at the same time.
Masonry buildings are very subject to damage during earthquakes because while they’re strong under compression, they easily break apart. This is why composite materials are now being used extensively in earthquake-prone areas. For example, reinforced concrete, with steel rods going up through concrete, adds shear strength. Also, in a wood buildings, plywood can be applied.
Common Questions about Strength as Basic Property of a Material
There are three types of strength. Compressive strength occurs when something is pressed or pushed up; tensile strength occurs when something is pulled; and shear strength occurs when something is twisted.
Brick is a strong material that is very resistant to pressure which means that it has compressive strength.
Masonry buildings are weak against earthquakes because while they’re strong under compression, they easily break apart. This is because they don’t have all three types of strengths.
